Electrochemical machining apparatus and method

ABSTRACT

A method of and an apparatus for the electrolytic machining of a conductive workpiece with a tool electrode spacedly juxtaposed therewith. Machining is carried out with a succession of machining pulses which render the workpiece anodic to solubilize workpiece material in the electrolyte filling the gap. The gapvoltage/gap-current ratio is ascertained at each pulse and, upon deviation from a predetermined value, is used to control the power supply, the electrode supply to prevent further deviation and prevent discharge across the gap during subsequent pulses.

United States Patent Inoue Reissued Sept. 30, 1975 [54] ELECTROCHEMICALM INING 3,202,599 8 19 5 Shierholt 204/228 A P TUS A METH D 3,275,5389/1966 Haupt fit 81.. 204/228 P ARA ND 0 3,288,693 11/1966 Livshits204/12925 [76] Inventor: Kiyo hi Inoue, 182. 3-c m 3,301,776 1/1967Hughes 204/228 Tamagawayoga, Setagayaku, 3,328,279 6/1967 Williams eta1. 204/228 TokyoJapan 3,332,864 7/1967 Woods 204/129.25

- I 3,365,381 1/1968 Fromsonn. 204 12925 [2 1 Flledl 1971 3,401,1029/1968 Stiff 204/228 3,533,927 10/1970 Manning 204/129.2

Appl. No.: 188,470

Related U.S. Patent Documents Primary Examiner-F. C. Edmundson Attorney,Agent, or Firm-Karl F. Ross; Herbert 1 ABSTRACT A'method of and anapparatus for the electrolytic machining of a conductive workpiece witha tool electrode spacedly juxtaposed therewith. Machining is carried outwith a succession of machining pulses which render the workpiece anodicto solubilize workpiece material in the electrolyte filling the gap. Thegap-voltage/gap-current ratio is ascertained at each pulse and, upondeviation from a predetermined value, is used to control the powersupply, the electrode supply to prevent further deviation and preventdischarge across the gap during subsequent pulses.

13 Claims, 2 Drawing Figures Reissue of: [64] Patent No.: 3,527,686Dubno Issued: Sept. 8, 1970 Appl. No.: 511,827 [57] Filed: Dec. 6, 1965[S2] U.S. C1. 204/129.2; 204/129.25; 204/225; 7 204/228 [51] Int. ClB23p l/02; 323p 1/14 [58] Field of Search 204/129.2, 129.25, 228,204/224, 225

[56] References Cited UNITED STATES PATENTS 2,717,326 9/1955 Gunton314/69 2,809,319 10/1957 Steele et a1. 314/69 2,826,540 3/1958 Keelfric204/228 3,097,252 7/1963 Robinson 13/33 I S 11 4.. I P l I Hi.- a i an22 ,,2 b 11- 23 J1 236T 20 I 0 1 1 and IPIL 27 1"- Reissued Sept.30,1975

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mvsnrron KIYOSHE INOUE ATTORNEY ELECTROCHEMICAL MACHINING APPARATUS .ANDMETHOD Matter enclosed in heavy brackets appears in the The presentinvention relates generally to electrochemical machining apparatus and,more particularly, to an electrical control' system therefor.

in electrochemical machining, the workpiece which is electricallyco'nductive is "spaced'from the tool electrode across a gap t'o which anelectrolytic liquid is supplied at predetermined pressure and inpredetermined quantity. i

An electrical power supply is connected across the machining gap toremove material from the workpiece. The process is basically that ofanodic dissolution of the workpiece in conformance with Faradays law.

In the typical'electr'ochemical machining apparatus, the followingmeasures are taken; insulation of the surfaces other than the machiningface of the machining electrode; maintenance of the machining electriccurrent density at'a constant valuei maintenance of the feed speed ofthe machining electrode at a constant value; and reduction ofthemachining time by accelerating the machining speed.

However, with acceleration of the electrochemical machining process, themachining gap formed between the electrode and the workpiece issometimesaccompanied by anelectro-discharge and at the same time, byv

such detrimentalphen'omena as short-circuiting of the gap with resultantdamage to electrode, workpiece or both. The measureswhich have beentaken hitherto to avoid these conditions were to maintain control of theservo f ee'd system providing relative approach of the electrode andworkpiece, to' maintain control of the electrical power supply and tocontrol the flow rate and pressure of the electrolytic liquid suppliedto the gap. However, these controlling functions take effect after thedetection of decrease of the machining gap voltage ortransient increaseofthe machining current following the actual generation of thee'lectro-discharge in the machining gap. Accordingly, it takes some timebefore i the controlling procedures limited by a certain time constanttake effect and it has been impossible these methods to prevent theelectro -discharge or shortcircuit condition from occurring.

It is the purpose of the present invention to' control machiningconditions so asto reduce the likelihood of occurrence of theabove-mentioned 'electro-discharge current or short-circuit current bydetecting the particular transient tendency, at an early stage prior tothe time when the machining gap indicates the'transition to suchelectro-discharge or short-circuit and to prevent its occurrence. i

It is an object of my invention to provide animproved electrical controlsystem for electrochemical machining in which signals are derivedrepresentative ofgap current and gap voltage respectively. These signalsare furnished to a control means which is operable responsive todeviations'of these signals from a predetermined relationship toinitiate a corrective control over a suitable element of the apparatusto prevent theoccurrence of gap short circuit condition.

- Otherobjects and advantages of the present invention will becomeapparent from the following description taken in connection with theaccompanying drawings, in which: I

FIG.:,1 is a schematic drawing'showing an embodimentof thepresentinvention; and

FIG. 2 is avoltage-current characteristic curvejllus tratin'g theprinciple-of operation relation to the-present invention. Y

With-reference to FIG. 1, a machining electrode 11 is shown. which isjuxtaposed with a workpiece 12. Within .the'machining gap'thus formed,an electrolyte liquid iscirculated under pressure through a passage 11aformed inside the electrode 11. The electrolyte supply 13 comprises afiltering apparatus and storage tank. A liquid-supply pump 13a'isprovided with an electromagnetic valve 13b whichc'ontrols the flow rateof the electrolyte liquid to be supplied to the gap.

The numeral -14 represents an electrode servo feed system including anelectric motor for maintaining the spaced gap relationship betweenelectrode 11 and workpiece 12. In the particular example of apparatusillustrated, the motor control winding is connected at one terminal tothe wiper of a potentiometer 15 which is connected across the machininggap. A reference voltage is provided at the otherterminal by a DC-source l6 connected across potentiometer 17 whose wiper is' tied to thisother terminal. Responsive to the voltage difference between points15'aand 17a, the opsecondary circuit connected in the opposite direction asshown'in'my copending U.S. application Ser; No.

316,95'5,filedon Oct. 17, 1963, entitled Ion Control System forElectrochemical Machining. Terminals 19 are inputterminals connectibleto an alternating cur- I rent power source. An electrical power sourcecontrolling switch 20 is connected as shown for a purpose. which will beexplained hereinafter. A high-frequency electric power source 21 isprovided including ahigh frequency coil 21a operatively connected to aninductance coil 23a of a series connected resonance circuit 23consisting of the above mentioned inductancecoil 23a connected in serieswith a capacitor 23 across the machining gap by means of a switch 22. Itwill be seen that the above mentioned low frequency direct currentpulse-type source 18 has superimposed thereacross a high frequencyelectrical waveform.,The numeral 24 represents a magnetic amplifierwhich is provided with a main winding 24a connected in series with arectifier 27 having output terminals 26. Magnetic amplifier 24 furtherhas an electriccurrent sensing winding 24b, a voltage sensing winding24c, and an iron core 24d. The above mentioned current sensing winding24b is con nected across a resistor 28, which resistor is in series withmachining power supply 18 and the machining gap. Voltagesensing winding24c has its terminals connected across the gap to sense gap voltage.

FIG. 2 represents characteristic curves of the machiming. voltage Vversus machining current 1 which may be expressed as machining currentdensity an a./cm.

In FIG. 2, the closed curve A shown represents the status in the normalcondition of operation when the machining gap is maintained at thepredetermined dirnension and, at the same time, the electrolyte fluid isprovided at the predetermined flowing rate and quantity through the gap.The products of the anodic dissolution i.e. workpiece particles arebeing removed at the predetermined rate thus maintaining thepredetermined electrical resistance in the machining gap. The

electrical power pulse supplied from the pulse generating apparatus 18just matches this predetermined value. Coincidental with the beginningof the power pulse of the pulse, the curve A starts from the originpoint and develops into a closed curve as shown. When there is a suddenchange in the dimension of the machining gap as caused, for example, bythe electrode feeding apparatus 14 or by a variation in flow rate andquantity of the supplied electrolyte liquid, the abovementioned curve Achanges to a closed curve B or C showing a drastic change of themachining voltage V versus machining currentl characteristic curvewithin the machining gap.

Namely, curve B shows a sharp upward turn at a point b with an abruptrise of the VI] characteristic. This indicates that, in spite of aslight change in the machining current I, the change of the machiningvoltage V tends to sharply increase. Further, the V/l characteristiccurve of this kind is attributable to the fact that when the machininggap exceeds the predetermined dimension, there is increased voltage dropdue to the electrolytic resistance of the machining gap which may tendto electro-discharge between workpiece and electrode. When the machininggap narrows below the predetermined dimension, gasification of theelectrolytic liquid within the machining gap may be caused by means ofelectrolysis or of generation of the Joules heat. This likewise tendstoward increased gap resistance and further exhibits a tendency towardelectrodischarge between the electrode and the workpiece V with possibledamage resulting.

Further, it is apparent that the curve C showing the characteristic V/Icurve is located between curves A and'B showing the intermediatecondition. However, if this particular condition is allowed to continue,it appears very likely that the characteristic curve of VII of themachining gap will tend to the condition of curve B. In other words,curves B and C reveal clear indication that the normal machiningcondition has transferred to abnormal machining condition. Otherwisestated, the individual functioning of the machining power source, servofeed apparatus and liquid supply apparatus are transferring from thepredetermined balanced condition to the unbalanced condition. Thisunbalanced condition is apparent when the V/I characteristic curve sochanges that the machining voltage V machining gap beyond the limit ofthe voltage level V which is provided with a precautionary safetyallowance at the stage prior to transfer to the predetermined generationof the electro-discharge phenomenon, it is necessary to detect thisparticular extreme condition and'to control, at least one of theelements, namely, the electrolytic liquid supply apparatus 23, servofeed apparatus 14 or machining pulse forming apparatus 18, to make theV/I characteristic curve within the machining gap to return to thepredetermined curve A. Depending upon the circumstances, it may benecessary to open the electric switch 20 of the electric power source tosuspend the supply of the machining electric power or to stop theoperation of the servo feed apparatus 14 by opening the switch 17b.

' DESCRIPTION OF OPERATION The present invention has been created toprovide means to detect such abnormal condition within the machining gapbefore damage has occurred. With reference to FIG. 1, the magneticamplifier 24 includes control windings 24b and 24c, which windings areexcited by the machining electric current and the machining electricalvoltage, respectively, in order to obtain the controlling signals basedupon the abovementioned detection, the turns ratio of the abovementionedwindings 24b and 24c is such that when the characteristic curve of themachining voltage versus the machining current within the machining gap,remains within the range of the above-mentioned curve A or the curve C,the excitation of the iron core 24d by the windings 24b and 24c isbalanced. Accordingly, the difference in excitation remains negligiblysmall so that the control signal output from the control output terminal26 in this case is extremely low. On the other hand, when the V/Icharacteristic curve within the machining gap rises toward the voltage Vas is the case with the curve B, then it is so arranged that thepredetermined control signal output is obtained from the terminals 26.

When the predetermined control signal output is obtained from theterminal 26, because of irregularities found in the predeterminedequilibrium conditions between power pulse output, electrolyte flow andservo feed, it is necessary to control the function of at least one ofthe above-mentioned elements. The procedure to be conducted at this timeis to suspend temporarily the supply of the machining electric pulsepower by means 'of opening, as mentioned above, the switch 20 or'theswitch 22. Alternatively, the feeding operation of theelectrode 11 meansmay be interrupted by suspending the operation of the feed motor 14through the operation of switch 17b or control may be exercised over theelectromagnetic valve 13b operatively connected in the electrolyte fluidsupply in order to increase the flow rate and quantity of theelectrolyte liquid supplied to the gap.

It will thus be seen that I have provided a novel apparatus and methodof control circuitry for electrochemical machining which controlcircuitry is operable in a timely manner to detect abnormal gapconditions and to prevent damage from occurring. While only a singleembodiment of the present invention has been illustrated and described,it will be apparent toone skilled in the art that various changes andmodifications may be made therein without departing from the spirit ofthe invention.

I claim:

1. An apparatusfor electrochemically machining a conductive workpiece,comprising: r I

a tool electrodespacedly juxtaposed with said workpiece across amachining gap; r means for relatively displacing said workpiece saidtool electrode to" maintain said gap; means for introducinga stream ofelectrolyte to said p; i a power supply connected to said'gap to providesuc cessive machining pulses thereto while rendering said workpieceanodic for electrolytic solubilization thereo-f' duringsaid pulses;means for deriving a'signa] representative ofgap current; means forderiving a signal representative of gap voltage; and i 1 -means'-forcontrolling the output from said power supply to said gap responsive toderivation of the ratio of said signals and the gap voltage/gap currentratiofrom a predetermined value. 2. The combination defined in claim 1-whereinthe last-mentioned means comprises a magnetic amplifier having apair-of control coils one of said coils being operatively connectedacrossvsaid gap for sensing gap voltage, andthe other of said coilshaving its terminals connected across a resistor in series between saidpower supply and the gap for sensing gap current.

3 An apparatus for electrolytically machining a conductive workpiececomprising:

a tool electrode spacedly juxtaposed with said workpiece acrossamachining gap; means for relatively displacing said workpiece and saidtool electrode to maintain said gap;

tmeans for introducing a stream of electrolyte to said flow-controlrneans connected between said fluid source and said gap; I 7

means for deriving a signal representative of g ap voltage; r

7 means for derivinga signal representative of gap current; and l Vmeans operatively connected to and controlling the i operation of saidflow-control means for increasing fluid flow to saidgap in response todeviation of the ratio of said signals and the gap voltage/gap currentratio from a predetermined value.

4. An apparatus for electrolytically machining a con ductive workpiececomprising:

a tool electrode spacedly juxtaposed with said workpiece across amachining gap;

means for relatively displacing said workpiece and said tool electrodeto maintain said gap;

means for introducing a stream of electrolyte to said a power supplyconnected to said gap to provide electrical machining pulses theretowhile rendering said workpiece anodic for electrolytic solubilization ofsaid workpiece in the electrolyte in said gap during said machiningpulses, said means for and relatively displacing said electrode and saidworkpiece including servo feed means for providing relative movementbetween said electrode and work- "piece; means for deriving a signalrepresentative of gap current;

means for deriving a signal representative of gap volt-,

age; and"''" means operatively connected to said servo feed I means forcontrolling said servo feed means responsive to,deviation of the ratioof said signals and the gap voltage/gapcurrent ratio froma predeterminedva1 ue. I, i .1 5. Anrapparatus for electrolytically machining aconductive workpiece comprisingz i a toollelectrode spacedly juxtaposedwith said workpiece across a machining gap; means for relativelydisplacing said workpiece and said tool electrode to maintain said gap;means for introducing a stream of electrolyte to said p; 4 Y I a powersupplyconnected tosaid gap to provide electrical machining pulsesthereto while rendering said workpiece anodic for electrolyticsolubilization thereof during said pulses; lrneans for deriving a signalrepresentative of gap rent; means for deriving-a signal representativeof gap voltage; f switching meansconnected ply and said'gap; and meansfor activating said switching means for interrupting power from saidsupply in response to deviati'on'of the ratio of said signals and thegap voltage/gap current ratio from a predetermined value. 6. An.apparatusfor electrolytically machining a conductive workpiece,comprising:

a tool electrode'spacdly juxtaposed with said workcurbetween said powersuppiece across a machining gapi means for relatively displacing saidworkpiece and said tool electrode to maintain'said gap; means forintroducing a stream of electrolyte to' said a power supply connected tosaid gap for providing electrical machining pulses thereto whilerendering said workpiece anodic for electrolytic solubilization thereofduring said pulses, said means forintroducing said electrolyte to saidgap including asourc'e of pressurized electrolyte connected to said gapto provide electrolyte flow thereto, and

flow-control means connected between said source and said gap;

means for deriving a signal representative of gap voltage;

means for deriving a signal representative of gap current; and

means operatively connected to and controlling the operation of saidflow-control means for varying selectively the electrolyte flow to saidgap in response to deviation of the ratio of said signals and the gapvoltage-gap current ratio from a predetermined value.

7. An apparatus for electrolytically machining a conductive workpiece,comprising:

a tool electrode spacedly juxtaposed with said workpiece across anelectrolyte filled gap;

servo feed means for maintaining said workpiece and electrode insubstantially constant spaced relationship during machining;

power-supply means for providing electrical machining pulses to said gapwhile rendering said workpiece anodic for electrolytic solubilizationthereof duringsaid pulses;

electrolyte-supply means for furnishing electrolyte fluid flow to saidgap;

a sensing network for deriving a signal representative of gap current;

a sensing network for deriving a signal representative of gap voltage;and I circuit means for comparing the ratio of said signals and forcontrolling the operation of at least one of said servo feed means, saidpower-supply means and said electrolyte supply means responsive tovariations in the relative magnitude of said signals and the gapvoltage/gap current ratio from a preselected relationship. 7

8. An apparatus for electrochemically machining a conductive workpiece,comprising:

a tool electrode spacedly juxtaposed with said workpiece across amachining gap; means for relatively displacing said workpiece and I saidtool electrode to maintain said gap; means for introducing a stream ofelectrolyte to said p V a'power supply connected to said gap to providesuccessive electrical machining pulses thereto while rendering saidworkpiece anodic for electrolytic solubilization thereof during saidpulses; means for controlling the output from said power supply to saidgap; said means including a magnetic amplifier including a pair ofcontrol coils and an output coil, one of said control coils connectedacross said gap for sensing gap voltage, and a resistor connected in series between said power supply and said gap, the other of said controlcoils connected across said resistor for sensing gap current; V

and meansoperatively connected to the output of said; power supply,adapted to control said output and the gap voltage/gapcurrent ratio froma predetermined value. t t 9. In an electrochemical device wherein metalis rerelative to the workpiece, the method of preventing damage to thetool or workpiece from short circuiting and the development of a faultcurrent across the gap between the tool and the workpiece whichcomprises detecting the condition of a momentary drop in current at theelectrodes prior to the development of a fault current to detect theimminence of a subsequent current surge as the fault current developsacross the gap, and using this detected momentary drop in current at theelectrodes to ac: tuate an auxiliary control circuit to cut off thepower supply.

I O. In an electrochemical process wherein a tool and a workpiecefunction as electrodes and the tool is advanced relative to theworkpiece to maintain a gap between the tool and the workpiece, a powersupply, an auxiliary control circuit to shut ofi the power supply, the

- method of preventing damage to the tool or workpiece by heat resultingfrom short circuiting across the gap which comprises detecting ,theimminence of the impending short circuiting by detecting a reduction incurrent-at the electrodes which is a condition that is symptomatic of animpending short circuit, converting the reduction of current to adifference in voltage signal, and amplifying the converted signal fromthis symptomatic condition to actuate the auxiliary control circuit toshut off the power supply! II.- In an electrochemical device wherein atool and a workpiece function as electrodes and the tool is advancedrelative to the workpiece to maintain a gap between the tool and theworkpiece, a power supply, an auxiliary con trol circuit to shut ofl thepower supply to prevent damage to the tool or workpiece by heatresulting froms horting across the gap, said control circuit comprising:

a. detector means connected to said power supply for detecting a,momentary decrease in the current which is symptomatic of animpending'short circuit, and VI). differential amplifier means receivingthe output of I said detector means and generating a difference voltagesignal in response to the detected momentary decrease which issufficient to actuate the control circult to cut off the power supply.I2. A control circuit as recited in claim 11 further comprisingoscillator means connected between said power supply and said detectorand producing an AC output voltage modulated by a change in the currentin said moved by electrolysis in a process wherein a tool and a I aminute gap between the tool and the workpiece, a

power supply for the electrodes and to advance the tool power supply,said detectormeans being responsive to said modulated AC voltage.

13; A control circuit as recited in claim 11, further comprising DCamplifier means, connected to the output of said differential amplifiermeans for amplifying said difference voltage signal.

1. An apparatus for electrochemically machining a conductive workpiece,comprising: a tool electrode spacedly juxtaposed with said workpieceacross a machining gap; means for relatively displacing said workpieceand said tool electrode to maintain said gap; means for introducing astream of electrolyte to said gap; a power supply connected to said gapto provide successive machining pulses thereto while rendering saidworkpiece anodic for electrolytic solubilization thereof during saidpulses; means for deriving a signal representative of gap current; meansfor deriving a signal representative of gap voltage; and means forcontrolling the output from said power supply to said gap responsive toderivation of the ratio of said signals and the gap voltage/gap currentratio from a predetermined value.
 2. The combination defined in claim 1wherein the last-mentioned means comprises a magnetic amplifier having apair of control coils, one of said coils being operatively connectedacross said gap for sensing gap voltage, and the other of said coilshaving its terminals connected across a resistor in series between saidpower supply and the gap for sensing gap current.
 3. An Apparatus forelectrolytically machining a conductive workpiece comprising: a toolelectrode spacedly juxtaposed with said workpiece across a machininggap; means for relatively displacing said workpiece and said toolelectrode to maintain said gap; means for introducing a stream ofelectrolyte to said gap; a power supply connected to said gap to provideelectrical machining pulses thereto while rendering said workpieceanodic for electrolytic solubilization thereof during said pulses, saidmeans for introducing electrolyte to said gap including a source ofpressurized electrolyte fluid connected to said gap to provideelectrolyte-fluid flow therethrough, and flow-control means connectedbetween said fluid source and said gap; means for deriving a signalrepresentative of gap voltage; means for deriving a signalrepresentative of gap current; and means operatively connected to andcontrolling the operation of said flow-control means for increasingfluid flow to said gap in response to deviation of the ratio of saidsignals and the gap voltage/gap current ratio from a predeterminedvalue.
 4. An apparatus for electrolytically machining a conductiveworkpiece comprising: a tool electrode spacedly juxtaposed with saidworkpiece across a machining gap; means for relatively displacing saidworkpiece and said tool electrode to maintain said gap; means forintroducing a stream of electrolyte to said gap; a power supplyconnected to said gap to provide electrical machining pulses theretowhile rendering said workpiece anodic for electrolytic solubilization ofsaid workpiece in the electrolyte in said gap during said machiningpulses, said means for relatively displacing said electrode and saidworkpiece including servo feed means for providing relative movementbetween said electrode and workpiece; means for deriving a signalrepresentative of gap current; means for deriving a signalrepresentative of gap voltage; and means operatively connected to saidservo feed means for controlling said servo feed means responsive todeviation of the ratio of said signals and the gap voltage/gap currentratio from a predetermined value.
 5. An apparatus for electrolyticallymachining a conductive workpiece comprising: a tool electrode spacedlyjuxtaposed with said workpiece across a machining gap; means forrelatively displacing said workpiece and said tool electrode to maintainsaid gap; means for introducing a stream of electrolyte to said gap; apower supply connected to said gap to provide electrical machiningpulses thereto while rendering said workpiece anodic for electrolyticsolubilization thereof during said pulses; means for deriving a signalrepresentative of gap current; means for deriving a signalrepresentative of gap voltage; switching means connected between saidpower supply and said gap; and means for activating said switching meansfor interrupting power from said supply in response to deviation of theratio of said signals and the gap voltage/gap current ratio from apredetermined value.
 6. An apparatus for electrolytically machining aconductive workpiece, comprising: a tool electrode spacedly juxtaposedwith said workpiece across a machining gap; means for relativelydisplacing said workpiece and said tool electrode to maintain said gap;means for introducing a stream of electrolyte to said gap; a powersupply connected to said gap for providing electrical machining pulsesthereto while rendering said workpiece anodic for electrolyticsolubilization thereof during said pulses, said means for introducingsaid electrolyte to said gap including a source of pressurizedelectrolyte connected to said gap to provide electrolyte flow thereto,and flow-control means connected between said source and said gap; meansfor deriving a signal representative of gap voltage; means for derivinga signal representative of gap current; and MEANS operatively connectedto and controlling the operation of said flow-control means for varyingselectively the electrolyte flow to said gap in response to deviation ofthe ratio of said signals and the gap voltage-gap current ratio from apredetermined value.
 7. An apparatus for electrolytically machining aconductive workpiece, comprising: a tool electrode spacedly juxtaposedwith said workpiece across an electrolyte filled gap; servo feed meansfor maintaining said workpiece and electrode in substantially constantspaced relationship during machining; power-supply means for providingelectrical machining pulses to said gap while rendering said workpieceanodic for electrolytic solubilization thereof during said pulses;electrolyte-supply means for furnishing electrolyte fluid flow to saidgap; a sensing network for deriving a signal representative of gapcurrent; a sensing network for deriving a signal representative of gapvoltage; and circuit means for comparing the ratio of said signals andfor controlling the operation of at least one of said servo feed means,said power-supply means and said electrolyte supply means responsive tovariations in the relative magnitude of said signals and the gapvoltage/gap current ratio from a preselected relationship.
 8. Anapparatus for electrochemically machining a conductive workpiece,comprising: a tool electrode spacedly juxtaposed with said workpieceacross a machining gap; means for relatively displacing said workpieceand said tool electrode to maintain said gap; means for introducing astream of electrolyte to said gap; a power supply connected to said gapto provide successive electrical machining pulses thereto whilerendering said workpiece anodic for electrolytic solubilization thereofduring said pulses; means for controlling the output from said powersupply to said gap; said means including a magnetic amplifier includinga pair of control coils and an output coil, one of said control coilsconnected across said gap for sensing gap voltage, and a resistorconnected in series between said power supply and said gap, the other ofsaid control coils connected across said resistor for sensing gapcurrent; and means operatively connected to the output of said powersupply, adapted to control said output in response to deviation of theratio of said signals and the gap voltage/gap current ratio from apredetermined value.
 9. IN ELECTROCHEMICAL DEVICE WHEREIN METAL ISREMOVED BY ELECTROLYSIS IN A PROCESS WHEREIN A TOOL AND A WORKPIECEFUNCTION AS ELECTRODES, AND THE TOOL IS ADVANCED RELATIVE TO THEWORKPIECE AT A RATE CORRESPONDING WITH THE RATE OF REMOVAL OF METAL FROMTHE WORKPIECE TO MAINTAIN A MINUTE GAP BETWEEN THE TOOL AND THEWORKPIECE, A POWER SUPPLY FOR THE ELECTRODES AND TO ADVANCE THE TOOLRELATIVE TO THE WORKPIECE, THE METHOD OF PREVENTING DAMAGE TO THE TOOLOR WORKPIECE FROM SHORT CIRCUITING AND THE DEVELOPMENT OF A FAULTCURRENT ACROSS THE GAP BETWEEN THE TOOL AND THE WORKPIECE WHICHCOMPRISES DETECTING THE CONDITION OF A MOMENTARY DROP IN CURRENT AT THEELECTRODES PRIOR TO THE DEVELOPMENT OF A FAULT CURRENT AT THE ELECTRODESPRIOR TO THE DEVELOPMENT RENT SURGE AS THE FAULT CURRENT DEVELOPS ACROSSTHE GAP, ANDUSING THIS DETECTED MEMENTARY DROP IN CURRENT AT THEELECTRODES TO ACTUATE AN AUXILIARY CONTROL CIRCUIT TO CUT OFF THE POWERSUPPLY.
 10. In an electrochemical process wherein a tool and a workpiecefunction as electrodes and the tool is advanced relative to theworkpiece to maintain a gap between the tool and the workpiece, a powersupply, an auxiliary control circuit to shut off the power supply, themethod of preventing damage to the tool or workpiece by heat resultingfrom short circuiting across the gap which comprises detecting theimminence of the impending short circuiting by detecting a reduction incurrent at the electrodes which is a condition that is symptomatic of animpending short circuit, Converting the reduction of current to adifference in voltage signal, and amplifying the converted signal fromthis symptomatic condition to actuate the auxiliary control circuit toshut off the power supply.
 11. In an electrochemical device wherein atool and a workpiece function as electrodes and the tool is advancedrelative to the workpiece to maintain a gap between the tool and theworkpiece, a power supply, an auxiliary control circuit to shut off thepower supply to prevent damage to the tool or workpiece by heatresulting from shorting across the gap, said control circuit comprising:a. detector means connected to said power supply for detecting amomentary decrease in the current which is symptomatic of an impendingshort circuit, and b. differential amplifier means receiving the outputof said detector means and generating a difference voltage signal inresponse to the detected momentary decrease which is sufficient toactuate the control circuit to cut off the power supply.
 12. A controlcircuit as recited in claim 11, further comprising oscillator meansconnected between said power supply and said detector and producing anAC output voltage modulated by a change in the current in said powersupply, said detector means being responsive to said modulated ACvoltage.
 13. A control circuit as recited in claim 11, furthercomprising DC amplifier means, connected to the output of saiddifferential amplifier means for amplifying said difference voltagesignal.